Older automatic transmissions used vacuum to decide when to shift - that's the kind I have. At some point they switched to electronic control of the shift points. You don't mention what year your car is (and I wouldn't know from that, anyway) but if it's vacuum controlled I'd look for vacuum leaks, if it's electronically controlled the only thing I can suggest is taking it in to a shop and have them diagnose it. On an electronic transmission it would probably be a sensor attached to the accelerator cable, but without the diagnostic equipment I couldn't even begin to guess where to start looking.